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[Daniel Krambrock]

This series of patches let the user choose a VMID suggestion strategy 
to avoid the recycled VMID problem.

Default is 'next-free', the previous strategy where the suggested VMID
is the lowest not used ID in range. This reuses VMIDs.
Added options are:
- 'max-1': selects the highest existing VMID and adds 1
- 'list': returns the lowest free VMID within the specified
  range that is not on a list of previously used VMIDs
On guest deletion the VMID is added to '/etc/pve/used_vmids.list'